In conventional test systems, for example, known immunoassay analyzers, a container 104, as shown in FIG. 1, sometimes referred to as a cuvette, will include a liquid sample 108. The liquid sample 108 usually only partially fills the container 104 and, therefore, presents a sample surface 112. Generally, the container 104 is closed with a cover or septum 116 that may be made from either a flexible material, for example, rubber or a type of foil, for example, a metallic seal.
In operation, referring now to FIG. 2, a pipette 204 is inserted through the cover/septum 116 in order to retrieve a portion of the sample 108. In some known systems, the pipette 204 is driven down into the sample 108 a set distance that is expected to reach, for example, all the way to the bottom of the container 104. This, however, has the drawback of coating the pipette 204 with sample material that then needs to be washed away in order not to contaminate successive pipettings from other containers 104. In addition, by moving the pipette a set amount, the possibility of either aspirating an empty container 104 or driving the pipette through the container, is not reliably mitigated.
In known conventional capacitive level sense techniques for identifying that the pipette is in contact with the fluid sample, there is a reliance upon a very small energy transfer from a capacitive probe to the object being detected, i.e., the sample surface 112. This approach, however, has a high rate of false positives, i.e., a false determination that the pipette 204 has reached the sample surface 112, as the technique responds to input from any surface on which the signal is present. Specifically, the conventional capacitive level sense systems do not work reliably with a container 104 having a septum or a foil seal closure.
What is needed, therefore, is a level detection system that accurately determines the location of the sample surface 112 in a container 104 having a septum or a foil seal closure.